Using a neat trick, you can turn any web browser window or tab into an instant notepad that you can write in. There really isn't much to this trick, and it works in just about web browser on just about every platform. All you need to do is copy and paste this into the URL bar, then hit Return:
data:text/html, <html contenteditable>
After you hit return the page will clear and you'll be able to start typing away. Paste in to your hearts delight, anything that is text based will go in, but images won't. The little scratch pad is completely ephemeral however, so don't expect to save anything from it directly and if you quit and relaunch the browser, your text will no longer be there (though the editable notepad will remain, just blank).
That little snippet can be bookmarked or saved as HTML, making it instantly accessible.